The present invention relates generally to a magnetic resonance (MR) imaging and, more particularly, to a non-intrusive system and method for de-icing a recondensor system and method of a liquid cooled superconducting MR magnet.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
To generate the necessary magnetic fields, high-field MRI magnets are utilized. The superconducting magnet, which is typically composed of wire, becomes a superconductor when cooled to a desired cryogenic temperature range. To achieve the desired cryogenic temperature, a cryogen is used to continuously cool the superconducting magnet. One common cryogen used for superconducting magnets is helium, which maintains a liquid state at approximately 4.2 degrees Kelvin (K). A bath of liquid helium is utilized to cool the superconducting wire so that the magnet can be energized or ramped to generate a desired magnetic field. The specific magnetic field achieved is a function of the number of turns in the wire, the ramp current, and the ramp voltage. Ideally, once the magnet is ramped, the magnet sustains the desired magnetic field until the magnet is ramped down. However, on occasion, the magnetic field is not sustained over the desired duration due to the temperature of the magnet rising above the temperature range necessary for the wire to act as a superconductor. In this case, the magnet quenches and the desired magnetic field is no longer generated.
To avoid quenching, closed loop or zero-boil-off cooling systems have been developed to continuously condense any helium that evaporates or boils-off. In a zero-boil-off cooling system, a constant helium level/volume is maintained within a magnet helium vessel, which houses the superconducting magnet, through the use of a recondensor that cools and liquefies boiled-off helium.
Under normal operating conditions, the magnet helium vessel operates at a pressure above atmospheric pressure to improve the efficiency of the zero-boil-off cooling system and prevent inflow of atmospheric air. Operating above atmospheric pressure is particularly important to prevent the ingress of atmosphere air because atmospheric air consists primarily of nitrogen and oxygen which have freezing temperatures of approximately 63 degrees K and 54 degrees K, respectively. Additionally, atmospheric air contains water vapor in the form of relative humidity, which has a freezing temperature of approximately 273 degrees K. As such, any air that enters (through a leak or unplanned opening) the magnet helium vessel freezes and, as such, may serve as an impediment to the zero-boil-off cooling system and interfere with maintaining the temperature of the liquid helium.
During operations which involve reducing the pressure within the magnet helium vessel to atmospheric pressure, the magnet helium vessel exists in a state which makes it particularly susceptible to air ingress. For example, during a magnet ramp or when filling the magnet helium vessel with liquid helium, the magnet pressure is brought to equilibrium with atmospheric pressure and then the magnet helium vessel is opened to allow feedthrough of ramp leads or a helium fill line. Through these operations, the potential exists for ice to build up in the reliquifier of the zero-boil-off system. If the reliquifier ices, the system will not recondense the boiled-off helium and the cooling system will cease to operate in zero-boil-off mode.
In such a case, excessive helium boil-off results and raises the pressure in the magnet helium vessel. Accordingly, a pressure relief valve is typically located outside the magnet helium vessel and is preset to open at a selected pressure that is greater than that of a normal operating pressure. When the magnet helium vessel pressure rises to the preset limit, the pressure relief valve opens to release the rising pressure at the expense of helium loss.
To rectify this situation, the MRI system typically must be removed from service to allow substantial servicing by field engineers to de-ice the cooling system. Specifically, the MRI apparatus must be powered down and a field engineer must open the cooling system via a bypass cooling loop and spray warm helium gas into the iced areas. Additionally, the recondensor, which serves to cool the evaporated helium back to a liquid, must be heated. However, upon opening the cooling system to purge it with warm helium gas, the potential exists to further contaminate the magnet helium vessel with air.
Furthermore, opening the cooling system to the atmosphere also usually results in a magnet quench when the ice clears from the cooling system and the injected warm helium gas comes in contact with the liquid helium and the superconducting magnet. As such, the magnet must again be ramped up (i.e. magnet coil re-energized) before operating the MRI apparatus.
It would therefore be desirable to have a system and method capable of de-icing a recondensor system of a liquid cooled superconducting MR magnet without potentially contaminating the magnet helium vessel with air. Additionally, it would be desirable to have a system and method to de-ice a recondensor system of a liquid cooled superconducting MR magnet without the risk of quenching the magnet.